Platelet-activating factor (PAF) is a pro-inflammatory lipid which is a potent inducer of vascular permeability that has been implicated as an essential component of pre-implantation stages of pregnancy. Both embryo- derived and uterine PAF have been suggested as key regulators for establishment of pregnancy. However, these claims are controversial. In some instances, published claims have been unable to be confirmed. We have established that PAF levels in rabbit endometrium change during early pregnancy and pseudopregnancy, and that rabbit endometrial membranes have high affinity binding sites for PAF. Furthermore, human endometrial stromal cells in culture make PAF, and their cellular content of PAF can be elevated by progesterone, and further amplified by prostaglandin E2 (PGE2). Conversely, in endometrial epithelial cells, PGE2 secretion is increased and PGF2alpha secretion inhibited by addition of PAF, suggesting the presence of a paracrine control mechanism between epithelial and stromal cells. Isolated rabbit endometrial cells also produce PAF in response to A23187 treatment, and there appears to be a difference in responsiveness between cells derived from day-6 of pregnancy and pseudopregnancy. We have further shown that a PAF antagonist CV3988 can inhibit implantation in rabbits, probably by an endometrial action, and that day-6 blastocysts respond to PAF in vitro by phospholipid turnover and increased intracellular calcium levels. The present research will determine the significance of PAG in the implantation process in rabbits. Various PAF antagonists, that compete or do not compete, with endometrial PAF receptors will be tested for ability to interfere with implantation when given at various times during pre- implantation pregnancy. Reversal of any such inhibition will be attempted by concomitant administration of a non-metabolizable PAF analog. These experiments will determine the specificity of this inhibitory action and at what stage of early pregnancy these antagonists are most effective. To define further the influence of these antagonists on the two components - embryo and endometrium - embryo transfer experiments of treated embryos to untreated pseudopregnant recipients and vice versa are proposed. To examine in more detail the physiological action of PAF on rabbit embryos, other experiments to study phospholipid turnover, second messenger generation and changes in intracellular calcium in response to PAF in embryos of various stages of development are proposed. Taken together, these experiments will define the role of PAF in the implantation process in rabbits and elucidate whether it is actions on the embryo or endometrium or both that are important for successful implantation. This information will be of importance for treatment of infertility due to failure of blastocyst growth and development and implantation, and for improving the success rate in patients requiring treatment with assisted reproductive technology.